1. Field of the Invention
This invention relates to the production of continuously cast metal strands through a mold, and more particularly to a method and an apparatus for continuously casting a defect-free strand by oscillating a mold supporting oscillating frame by an electrohydraulic or mechanical drive means.
2. Description of the Prior Art
In a continuous casting process, it is necessary to prevent the problems of seizure or breakout of the cast strand by reducing the friction between the mold and the cast strand. In this regard, it has been the general practice to resort to the so-called mold oscillating system which reduces the friction between a mold and a cast strand by oscillating the mold in the vertical direction during the casting operation.
Generally, in the casting operation using a mold oscillating system, the mold is oscillated such that the maximum speed of the downward movement of the mold is greater than the strand withdrawing speed. More specifically, as shown in the diagram of the downward mold speed versus the cast strand withdrawing speed of FIG. 1, the oscillation of the mold is set so that the ratio of the time length tn in which the downward speed of the mold is greater than the strand withdrawing speed to the time length tp of the downward mold movement (tn/tp..times.100) is in the range of 60% to 80%.
With regard to more specific conditions of oscillation, it has been the conventional practice to set the frequency of oscillation at 60-90 C/min and the amplitude of oscillation at 6-10 mm. However, under such conditions, there often occur positive and negative defective structures at the roots of the oscillation mark, which will invite fine cracks, in addition to the defects due to powder inclusion.
The oscillation defects which occur at the roots of the oscillation mark mainly exist in the surface layer within the depth of 2 mm, so that, if a cast strand is rolled into a sheet without any prior treatment, the defects come out as an irregular pickling pattern and surface defects, thus impairing the surface quality of the resulting steel sheet to a considerable degree. Therefore, according to the conventional procedure, these defects are removed by a grinding operation in the stage of intermediate products, which will obviously result in unacceptably high production costs due to the increase in expenses for defect removing treatment and reductions in yield.
In this connection, FIG. 2 shows the relationship between the rate (%) of occurrence of the oscillation defects of the cast strand and the frequency of oscillation C/min. As shown in the figure, it has been known that the rate of oscillation defects can be reduced by increasing the frequency of oscillation. However, the increase of frequency has been limited to a certain level since a high oscillation frequency has possibility of inducing so-called "sloshing", surfacewise oscillations of molten steel, as well as the resonance of the oscillation system of the oscillating frame at its natural frequency.